Recent advances in informatization have led to an increase in demand for data communication traffic in data communication and the like. Accordingly, there are network-related needs for broader bandwidths and lower operation costs.
Mobile telephone networks and the like utilize networks constructed by wireless links including Fixed Broadband Wireless Access (FWA) which is based on a wireless scheme utilizing a frequency such as a millimeter waveband that enables broadband transmission.
FIG. 1 is a graph illustrating a correlation between Signal to Noise Ratio (SNR) and the transmission rate of modulation modes.
In the graph illustrated in FIG. 1, an abscissa represents SNR and an ordinate represents bandwidth. A communication quality of a wireless link fluctuates according to an SNR of a reception signal. Adaptive modulation technology is drawing attention as means to realize a wider bandwidth for wireless links. Adaptive modulation technology involves adaptively finding and using a modulation scheme with the highest transmission efficiency based on a wireless status of a wireless link. Adaptive modulation technology enables optimal wireless communication corresponding to a wireless environment to be performed and frequency efficiency to be improved.
If the modulation mode to be used in the future by each wireless link can be predicted, the communication quality of traffic flowing through a network can be guaranteed. An example of a wireless link using adaptive modulation technology is disclosed in JP2006-505221A. As described in this document, since a wireless link using adaptive modulation technology may have a certain degree of correlation with past history, the modulation mode to be used by the wireless link can be predicted (estimated).
Furthermore, as disclosed in JP2004-363679A, the modulation mode can be predicted (estimated) by utilizing weather information.
An example of path control such as that performed in a wired network is disclosed in “Minimum interference routing with applications to mpls traffic engineering” (INFOCOM, 2000) by M. S. Kodialam et al. When path control disclosed in this document is applied to a network constituted by FWA using adaptive modulation technology, unlike when wired, adaptive modulation causes fluctuation in the a communication rate of a wireless link, resulting in congestion or the like and a deterioration of traffic communication quality.
One load distribution scheme by FWA utilizing adaptive modulation is disclosed in JP2006-313992A. This scheme involves having a wireless link in a plurality of routing paths distribute and relay traffic. Such a scheme enables traffic to be distributed and increases the utilization of an entire network.
However, with this scheme, each packet may potentially travel along a different path and a delay jitter or the like may occur. In this case, the scheme is unsuitable for traffic such as VoIP (Voice over IP) that requires a guarantee of communication quality. Such traffic requires path control in which traffic travels along the same path. In addition, using this scheme without modification makes it difficult to estimate what level of priority is sufficient to maintain traffic quality considering that paths cannot be designated, and to maintain and manage communication quality of preferred traffic.
An example of existing technology that takes path control into consideration is presented in “Qos-based adaptive modulation under rainfall environment in gigabit millimeter-wave broadband wireless access system” (IEEE RAWCON, 2003) by J. Sangiamwong et al. This document proposes a path control scheme by FWA having an adaptive modulation function. The scheme proposed in this document involves searching an optimal modulation mode and path for each flow. However, since the scheme provides path control based on the state of a wireless link at the present moment, whether or not the modulation mode can be maintained thereafter is not guaranteed. In addition, the overhead of selecting a modulation mode for each flow at each base station is disadvantageously high.
As described above, technology exists for performing path control by predicting or estimating the modulation mode to be used by a wireless link which varies due to adaptive modulation.